The present invention relates to a pager with a display and, more particularly, to a display pager of the type using liquid crystal or like display elements which are not themselves radiant, and which is capable of automatically illuminating the display as the external luminance becomes low.
Some modern pagers are capable of displaying data in numerals and messages to convey such information to the user. The prerequisite for this type of pager is that it be small in size and portable and, to meet this need, non-radiant liquid crystal elements which consume a minimum of power are widely used. Because these display elements are non-radiant as mentioned, such a pager is provided with a lamp for illumination and a switch associated with the lamp.
One approach to a smaller-size configuration is to replace the switch with a light-sensing photo-sensing phototransistor, as disclosed in Japanese Patent Application Ser. No. 58-178683 corresponding to U.S. patent application Ser. No. 654,082, now U.S. Pat. No. 4,644350, 2-17-87, EPC Patent Application No. 84 111 419.2, Canadian Patent Application No. 463,961 An example of the phototransistor scheme is shown in FIG. 1.
In the prior art pager shown in FIG. 1, phototransistor 10a is sensitive to external luminance and, when it is low, turns on a lamp 10b. When the lamp 10b is turned on, light issuing from the lamp 10b is diffused inside a diffuser plate 12 and transmitted sequentially through liquid crystal 14 and an acryl plate 16 as indicated by arrows, thereby allowing displayed data to be read by the user. A partition plate 18 serves to intercept a part of the light from the lamp 10b which is directed toward the phototransistor 10a.
However, although the partition plate 18 successfully prevents the light from the lamp 10b from becoming directly incident to the phototransistor 10a, it fails to intercept the light reflected by the acryl plate 16 as well as externally derived light. Therefore, when the external luminance is dim, the display cannot function in a positive manner. Specifically, when the luminance level is near the threshold level which will permit photosensor 10a to turn on the lamp, the output signal of the photosensor is susceptive to even insignificant changes in the ambient conditions and leakage of light issuing from the lamp 10b (as is indicated by phantom lines in FIG. 1) can cause the lamp 10b to repeatedly turn on and off at short intervals, rendering the displayed information difficult to see.